1. Technical Field
The present disclosure relates to a communication system, a communication method, a base station device, and a terminal device which perform beamforming training to determine beam patterns to use for communications.
2. Description of the Related Art
Millimeter-wave communications, which use radio signals in the millimeter-wave spectrum (60 GHz for example), are in widespread use in recent years. Wireless communication standards for millimeter-wave communications include, for example, Wireless Gigabit (WiGig), WirelessHD (wireless high definition), ECMA-387, IEEE 802.15.3c, and IEEE 802.11ad.
Radio signals in the millimeter-wave spectrum have such radio properties as strong linearity and high free-space path loss. Due to such properties, typical millimeter communications use beamforming to control the directivity of radio signals using multiple antennas.
Beamforming is a technique for causing a directional beam to follow the position of a communication partner by controlling the direction and width of the beam. For example, connection can be established with multiple communication partners (wireless communication devices) by using beamforming to switch the direction of a beam for each of the communication partners by time division.
IEEE 802.11ad is an example of millimeter-wave wireless communication standards setting forth the beamforming technique (see IEEE 802.11ad-2012 IEEE Standard for Information Technology-Telecommunications and Information Exchange between systems-Local and Metropolitan networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications Amendment 3: Enhancements for Very High Throughput in the 60 GHz Band (hereinafter referred to as Non-patent Literature 1)). Non-patent Literature 1 describes a method (beamforming protocol) for selecting a beam pattern used for communications from multiple beam patterns. Specifically, Non-patent Literature 1 describes selecting an optimal beam pattern for communication by causing wireless communication devices communicating with each other to transmit training frames containing information for selecting an optimal beam pattern to each other while changing beam patterns.
Further, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2012-507946 (hereinafter referred to as Patent Literature 1) describes an example of application of the beamforming technique. Specifically, in transmitting a training frame to a communication partner, a wireless communication device includes, in the training frame, part or all of a reception result of a training frame previously received from the communication partner so as to feed back the reception result.
In the beamforming protocol disclosed in Non-patent Literature 1, beam selection is made by a communication partner, not by a device that forms the beam patterns and transmits signals using the respective beam patterns (hereinafter referred to as a transmitter device). Specifically, a communication partner selects a beam pattern with the best received signal quality from beam patterns used to receive respective training frames and notifies the transmitter device of the ID of the selected beam pattern. On the other hand, in a high interference environment where, for example, multiple wireless communication devices exist around a transmitter device, it is preferable that the transmitter device select an optimal beam pattern considering the influence of the interference and the like. However, it is difficult for the transmitter device to know reception results of all training frames received by the communication partner and therefore to select an optimal beam pattern.
In the technique disclosed in Patent Literature 1, a communication partner feeds reception results of training frames back to the transmitter device. To do so, the communication partner includes the reception results in training frames and transmits the training frames. Consequently, the length of the training frames increases, leading to decreased communication efficiency.